I do agree with Todd about what is proper with an Argyle or Prince Charlie. I would wear the bow tie.
I do not know the BBC's rules of attire for the presenters. I do know that some media outlets have made the neck tie the mandatory wear on men in the media view of the public. That Robert was wearing a kilt, is probably fine as the rules are not as specific about the lower attire, as it is not often in the public view. For many years a presenter on BBC International sat behind the desk giving the news in a proper suit jacket, white shirt, and necktie. Not noticed by the public, was that he was in athletic costume and tennis shoes on the lower part. Only those that were in the studio ever saw the full display.
From what little I saw of Robert's attire, he could be wearing scrunched hose and sneakers, even though proper hose, garters, flashes and shoes would be what us "traditionalists" would expect to see with a PC.
Respect to all points of view,
Steve